1. Field of the Invention
The present invention generally relates to gain control circuits and, more particularly, to a gain control circuit for controlling an operation of a variable-gain amplifier or the like for use in wireless transmitters and receivers.
2. Description of the Related Art
The development of mobile wireless communication technology is occurring at a considerable growth rate. Mobile wireless communication occurs between a base station and a mobile station. When the distance between the mobile station and the base station changes, or when an obstacle to radio communication such as a building is located between the mobile station and the base station, the signal intensity at a wireless transmitter or a wireless receiver changes significantly.
In order for signals to be correctly transmitted and received in mobile communication, it is required and desirable that a change in signal intensity be controlled to be at a minimum level. For this reason, it is necessary to introduce power control whereby an amplifier gain is enlarged when a signal intensity is small and reduced when a signal intensity is large. A variable-gain amplifier capable of controlling a gain depending on the signal intensity should be provided in a mobile wireless communication system.
FIG. 7 is a circuit diagram showing a variable-gain amplifier (variable-gain cell) of a current switch type according to the related art. Referring to FIG. 7, the variable-gain amplifier includes an input terminal 71 for receiving an input signal at a voltage V.sub.in, an output terminal 72 for outputting an amplified or attenuated signal at a voltage V.sub.out, a current switch 73, a load 74 and a differential amplifier 75. A description will now be given of the operation according to the related art.
When the input signal at the voltage V.sub.in is supplied to the input terminal 71 in the variable-gain amplifier shown in FIG. 7, the amplified or attenuated signal at the voltage V.sub.out is output from the output terminal 72.
Whether the input signal at the voltage V.sub.in is amplified or attenuated depends on a gain performance of the amplifier controlled in accordance with a gain control voltage V.sub.ctrl0. The gain control voltage is fed to the current switch 73. The current switch 73 shunts a collector current of the differential amplifier 75 and supplies the shunted current to the load 74. Thus, the gain of the input signal at the voltage V.sub.in is controlled in accordance with the gain control voltage V.sub.ctrl0.
A voltage gain of the related-art variable-gain amplifier shown in FIG. 7 will now be calculated. An emitter current supplied to the differential amplifier 75 of the related-art variable-gain amplifier is proportional to an absolute temperature and inversely proportional to resistivity. Generally, the emitter current having such a characteristic is generated by a band-gap voltage generation circuit.
An ac output small-signal ic of the differential amplifier 75 in relation to the input signal voltage V.sub.in is given by the following equation ##EQU1## where q indicates an elementary electric charge, k indicates Boltzmann constant, T indicates an absolute temperature, K.sub.1 indicates a constant, and .rho. indicates a resistivity of a semiconductor chip in which the variable-gain amplifier shown in FIG. 7 is formed. The resistivity .rho. has temperature dependence.
The above equation (1) is used to determine the output voltage V.sub.out of the current switch 73 in relation to the gain control voltage V.sub.ctrl. Thus, ##EQU2## where K.sub.R is a constant. Referring to the equation (3), the gain control voltage V.sub.ctrl0 is set to satisfy the relation EQU exp((-q/kT).multidot.V.sub.ctrl0)&gt;&gt;1
The equation (2) above may be simplified as follows. ##EQU3##
From the equation (3) above, a voltage gain A.sub.v is given by the equation (4) below. ##EQU4##
The equation (4) shows that the voltage gain A.sub.v has temperature dependence such that it decreases as the temperature rises.
FIG. 8 is a graph showing the temperature dependence of a variable-gain amplifier according to the related art. For example, FIG. 8 shows that the voltage gain A.sub.v varies greatly as the temperature varies between 200K and 400K.
As described above, the variable-gain amplifier according to the related art has a drawback in that the voltage gain A.sub.v varies significantly as the temperature varies, presenting a problem in mobile communication. The gain control characteristic preferably remains unchanged regardless of a temperature variation. In other words, there is a demand for a variable-gain amplifier having a voltage gain A.sub.v not affected by a temperature variation.